This is the thing that people don't want to hear. There is no guaranteed, long-term store of value. Period. End of story. The closest is precious metals, especially the "monetary" precious metals, but even they are valued based on their usage (if society collapses, platinum & palladium will probably have their values go "poof", for instance) and fluctuate over time. They're one of the few goods that you can literally physically hold on to for decades and they at least won't tank to zero, but they still may not be worth "as much" as what you spent to get them.